Thiazole-5-Carboxylic Acid

In commercial fine chemical inventories, thiazole-5-carboxylic acid (CAS 14527-41-4, molecular formula C₄H₃NO₂S, molecular weight 129.14 g mol⁻¹) is listed as a heterocyclic building block supplied as a white to off-white crystalline powder. The ring numbering follows IUPAC convention with the sulfur atom occupying position 1 and the nitrogen at position 3; the carboxyl group is therefore located on the carbon adjacent to sulfur, giving a substitution pattern distinct from the more widely studied 2- and 4-carboxylic acid isomers. Batch-to-batch consistency relies on controlled crystallization from aqueous ethanol, and the material is routinely shipped in amber HDPE containers under nitrogen overlay to mitigate hygroscopicity and light-induced discoloration. A representative bulk specification, derived from a vendor certificate of analysis for a research-grade lot (purity ≥98%), is presented in Table 1.

A White Crystalline Solid: Physical Characterization and Quality Parameters

Differential scanning calorimetry (DSC) at 10 °C min⁻¹ under flowing nitrogen records a sharp endothermic melt with an onset temperature in the range 126–129 °C. Thermogravimetric analysis (TGA) up to 150 °C shows mass loss <0.3%, confirming the absence of lattice solvent. Water content determined by coulometric Karl Fischer titration (USP ⟨921⟩ Method Ia) is controlled at ≤0.5%; when ambient relative humidity exceeds 60%, the product is pre-dried in a vacuum oven at 40 °C and <10 mbar for a minimum of 4 h before use in water-sensitive transformations. Elemental impurity profiles are screened against ICH Q3D Guideline for Oral Bioavailability products, with lead, cadmium, and arsenic each below 5 ppm and Class 1 and 2A metals collectively not exceeding 20 ppm. Residual solvents are quantified by headspace GC-FID following ICH Q3C; the principal residual is ethanol at <500 ppm. Storage stability data collected on three consecutive lots stored at 2–8 °C over 24 months show no degradation >0.5% area by HPLC and no measurable increase in the 2,4-dicarboxy-thiazole contaminant.

Table 1. Commercial specification parameters for thiazole-5-carboxylic acid (representative lot).

Test	Method	Specification
Purity (HPLC, area%)	USP ⟨621⟩, C18, 254 nm	≥98.0%
Water (KF)	USP ⟨921⟩, Method Ia	≤0.5%
Residue on ignition	USP ⟨281⟩	≤0.1%
Heavy metals (as Pb)	USP ⟨233⟩	≤10 ppm
Appearance	Visual	White to pale yellow crystalline powder

When Employed as a Suzuki Coupling Substrate, Steric and Electronic Factors Favor 5-Position Substitution

Suzuki–Miyaura cross-coupling on halogenated thiazole-5-carboxylic acid scaffolds proceeds with regiochemical fidelity that distinguishes this isomer from thiazole-2-carboxylic acid. The carboxyl group in position 5 exerts a weak electron-withdrawing mesomeric effect that deactivates the ring toward oxidative addition at the adjacent 4-position but leaves the 2-position sufficiently electron-deficient to react under standard Pd(PPh₃)₄ catalysis. A typical transformation — 2-bromothiazole-5-carboxylic acid coupled with 4-methoxyphenylboronic acid in degassed 1,4-dioxane/water using 2 mol% Pd(PPh₃)₄ and 2 equivalents of K₂CO₃ at 85 °C for 16 h — delivers the biaryl product in 82–88% isolated yield after acidification and recrystallization from isopropanol. In contrast, 2-bromothiazole-4-carboxylic acid under identical conditions yields 43–51% due to competing proto-debromination and decarboxylation; the 5-acid remains intact because decarboxylation onset (by TGA) occurs at 195 °C, whereas the 2-acid loses CO₂ at 138 °C. This thermal margin allows reaction temperatures up to 110 °C without generating the unsubstituted thiazole byproduct. On a pilot-plant scale (a 20 L jacketed glass reactor, anchor stirrer at 180 rpm), the exotherm is controlled by dosing the boronic acid solution over 45 min while maintaining internal temperature at 82 ± 3 °C. In-process HPLC monitoring (Waters XBridge C18, 3.5 µm, 4.6 × 150 mm) shows complete conversion of the bromide within 12 h. The crude product is isolated by pH adjustment to 2.0 with 6 M HCl, cooling to 5 °C, and centrifugation; purity after drying is routinely >97% (HPLC). The 5-carboxyl group does not undergo significant protodecarboxylation under these conditions, a documented shortcoming of the 2-isomer that necessitates lower reaction temperatures and higher catalyst loadings.

Dissolution Behavior and Pre-formulation Considerations

The aqueous solubility of the free acid is pH-dependent. At 25 °C in deionized water, saturation solubility is 0.6 mg mL⁻¹; raising the pH to 7.4 (phosphate-buffered saline) increases solubility to 12 mg mL⁻¹ through carboxylate formation. The experimentally determined pKₐ of the carboxyl group, measured by potentiometric titration in 0.1 M NaCl at 25 °C, is 3.08 ± 0.05. For amide coupling reactions in anhydrous media, the compound is freely soluble in DMSO (>50 mg mL⁻¹), DMF (38 mg mL⁻¹), and THF (24 mg mL⁻¹). When preparing stock solutions for high-throughput chemistry, the solid is dissolved in dry DMF containing 3 Å molecular sieves; solutions are stable for 48 h at ambient temperature with <2% esterification by DMF decomposition products as confirmed by ¹H NMR. The sodium salt can be generated in situ by adding one equivalent of 1 M NaOH, facilitating aqueous-phase Suzuki couplings without an organic co-solvent. In centralized medicinal chemistry production facilities, thiazole-5-carboxylic acid is routinely converted to the corresponding acyl chloride using oxalyl chloride and catalytic DMF in dichloromethane at 0–5 °C, then coupled with primary or secondary amines to yield a library of amides. The 5-carboxamide moiety is a recurring motif in inhibitors of Rho-associated protein kinase (ROCK) and casein kinase 1 (CK1), where it engages the hinge-region backbone through a bidentate hydrogen-bond network. Unlike thiazole-4-carboxylic acid, the 5-isomer acyl chloride shows markedly slower hydrolysis: in a head-to-head experiment with 0.1 M HCl/THF/water (1:1 v/v) at 25 °C, the half-life of the 5-acyl chloride exceeded 120 min, while the 4-acyl chloride decomposed with a half-life of 18 min. This stability permits sequential addition of nucleophiles in a single pot; a representative procedure charges the pre-formed acid chloride to a slurry of the amine hydrochloride and 2.5 equivalents of N-methylmorpholine in THF at −10 °C, then warms to ambient temperature over 2 h. Work-up involves quenching with 10% aqueous citric acid and extraction, delivering the amide in 74–92% yield after silica gel chromatography. The enhanced solution stability of the 5-acyl chloride also enables safe scale-up in a 50 L enamel-lined reactor with brine jacket cooling; process safety evaluation using accelerating rate calorimetry indicates an exotherm onset above 100 °C, well outside the operating window, whereas the 2-isomer analogue begins vigorous decomposition at 64 °C. This operational boundary is critical when designing amidation cascades that require elevated temperatures for sluggish amines.

Steric vs. Electronic: Directing Metalation at the 2-Position

Directed ortho-metalation (DoM) strategies exploit the carboxylate anion as a weak directing group, but the thiazole-5-carboxylic acid system responds differently from the 4-isomer. Treatment of thiazole-5-carboxylic acid with 2.2 equivalents of lithium diisopropylamide (LDA) in THF at −78 °C initially deprotonates the acid and generates the lithium carboxylate; a second equivalent of LDA effects lithiation at the 4-position with a half-life for metalation of approximately 90 min at that temperature. Quenching with DMF (3.0 equivalents) followed by warming to −20 °C over 1 h gives 4-formylthiazole-5-carboxylic acid in 65–70% yield. Attempts to functionalize the 2-position by this route are unsuccessful because the C-2 hydrogen is acidified by the proximal nitrogen but the lithium carboxylate oxygen does not coordinate the metalating agent in a geometry conducive to abstraction. In contrast, thiazole-4-carboxylic acid under identical conditions delivers the 5-lithio species, enabling synthesis of 5-substituted-4-carboxylic acids. For selective 2-functionalization of the 5-acid, a halogen–metal exchange using 1.1 equivalents of n-BuLi on the corresponding 2-bromo derivative at −100 °C in diethyl ether is required; the resulting 2-lithio species is stable for <20 min before undergoing ring fragmentation. The divergent directing effects of the two carboxyl isomers are routinely exploited in the parallel synthesis of regioisomeric kinase inhibitor scaffolds: the 5-acid delivers 2-aryl-5-carboxamide analogues, while the 4-acid provides access to 5-aryl-4-carboxamide series.

Table 2. Comparative properties of isomeric thiazolecarboxylic acids.

Isomer	CAS	mp / °C	pKₐ (COOH)	log P	Decarboxylation onset / °C	Distinctive coupling behavior
2-Carboxylic acid	141-90-2	92–94	2.15	0.42	138	Rapid decarboxylation limits thermal Pd couplings
4-Carboxylic acid	14527-43-6	107–109	3.01	0.68	172	Directs metalation to C-5; acid chloride less stable
5-Carboxylic acid	14527-41-4	126–129	3.08	0.71	195	Directs metalation to C-4; robust under Suzuki conditions